Photoreception and sensory transduction will be investigated in the fungus Phycomyces, in connection with its phototropism and other blue light responses. The long term objective is to understand, at the cellular and molecular levels, the processes by which a stimulus such as light is transduced into a cellular response. The unicellular sporangiophore of Phycomyces serves as a model system for primary receptor cells. The absolute operating range of 10-10:1 in light intensity and the kinetics of light and dark adaptation are similar to those observed in visual photoreceptors; in Phycomyces, the cell is naturally isolated and amenable to genetic approaches. Action spectra for phototropism and the related light-growth response will be measured to characterize the low and high intensity photosystems of the sporangiophore. Experiments with monochromatic and dichromatic continuous irradiation will be applied in a way that allows the determination of components of each photosystem and their interaction with components of the other photosystem. These experiments will also involve single and double pulse stimuli applied to dark adapted sporangiophores of wild type and mutants with altered photoreceptors; the experiments will detect any light-absorbing intermediates. The experiments will be recorded with time-lapse video equipment. Null action spectra for the light-growth response will be measured automatically on the Phycomyces tracking machine; the results will be compared with phototropic balance spectra to determine whether these responses both use the components of the photoreceptor system in the same way. The recently discovered dependence of dark adaptation on low level residual light will be pursued to help establish which photoreceptor mediates this cryptic sensitivity when the sporangiophore is unable to respond directly. System-identification experiments on the light-growth response will be continued on the tracking machine with sum-of-sinusoids test stimuli to measure the dynamic and nonlinear aspects of this response under conditions of wavelength, temperature, and intensity range. These experiments will involve both wild-type and mutant strains, and will be interpreted with analytical models for the kinetics of the photosensory transduction chain.